0000000000370034
AUTHOR
Romain Piolet
Supercritical hydrothermal synthesis of ZnO nanopowders beyond the critical point : understanding of nucleation and growth steps
The supercritical hydrothermal synthesis of nanopowders (especially metal oxide) has been widely studied. To the best of our knowledge, no nanoparticle formation mechanism has been published yet. In this prospect, this study is dedicated to the understanding of metal oxide nanoparticle nucleation and growth mechanisms. For this purpose, zinc oxide is used as a model material. First, the influence of synthesis operating conditions such as pressure, temperature, pH, precursor concentrations and solution flow rates on particle morphological properties (size, particle size distribution or morphologies) has been investigated. Hence, two approaches have simultaneously been carried out. The first …
Influence of the pH on the ZnO nanoparticle growth in supercritical water: Experimental and simulation approaches
Abstract In order to improve the knowledge on the nucleation and the growth mechanisms of metal oxides nanoparticles produced in supercritical water domain, ZnO was used as a “model” material. A continuous process of hydrothermal synthesis was employed to synthesize ZnO nanopowders ( T = 410 °C and P = 305 bar) from Zn(NO 3 ) 2 and KOH solutions with different values of [KOH]/[Zn(II)] ratio from 0 to 8 in order to investigate the pH effect on the growth of ZnO nanocrystallite in terms of size and morphology. The samples were characterized by X-Ray Diffraction and Transmission Electronic Microscopy. ZnO crystal was considered as a cylindrical crystallite with a diameter D and height H . Es…
Thermodynamics of Nanoparticles: Experimental Protocol Based on a Comprehensive Ginzburg-Landau Interpretation
MATERIAUX+SMR:SDA; The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. On the basis of experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Ginzburg-Landau theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on …
CFD simulation of ZnO nanoparticle precipitation in a supercritical water synthesis reactor
International audience; Continuous hydrothermal flow synthesis process has shown great advantages concerning the control of particle size and morphology through the optimization of supercritical water processing parameters. In particular, micromixing is a key issue of the process for controlling the nucleation mechanism. A Computational Fluid Dynamics (CFD) model is suggested for nanoparticle size determination using a population balance approach. Models for reaction kinetics, thermodynamics, nucleation and growth are presented. The effects of base concentration and hydrodynamics are investigated. Results show that the CFD may be valuable simulation tool for controlling the size and the sha…
Hydrothermal growth of ZnO nanostructures in supercritical domain: Effect of the metal salt concentration (Zn(NO3)2) in alkali medium (KOH)
Abstract The metal salt concentration effect on the size and morphology of ZnO NPs was highlighted through its synthesis thanks to a continuous one-step method at 401 ± 15 °C and 306 ± 8 bar. Experiments were performed from Zn(NO3)2 and KOH as reactants in concentration ranges of 10–480 mM and 40–1920 mM, respectively. A constant [KOH]/[Zn(NO3)2] ratio of 4 was fixed in order to maintain a constant pH value between 12.5 and 13.0. The as-prepared NPs were characterized by X-ray diffraction and (high-resolution) transmission electron microscopy. Based on Louer's method, ZnO crystal exhibiting a hexagonal structure was considered as cylinder with a diameter D and a height H. The D/H parameter …
Original Supercritical Water Device for Continuous Production of Nanopowders
Well-crystallized ZnO, ZrO2, TiO2, CeO2, Y2O3 and La2O3 nanoparticles are synthesized under supercritical water conditions (T > 647 K and P > 22.1 MPa) using a home-made continuous process. At room temperature, metallic salts with or without aqueous hydroxide solution (KOH or NaOH) are pressurized to 25–30 MPa. Then, the reactant(s) is/are rapidly heated to 673–773 K by mixing with the supercritical water in a patented reactor. Residence time is in the range from 2 to 8 s. XRD, TEM and surface area analyses highlight the production of pure and well-crystallized nanoparticles with a uniform size distribution.
Hydrothermal Synthesis of ZnO Crystals from Zn(OH)2 Metastable Phases at Room to Supercritical Conditions
The originality of this work is to highlight the effect of temperature and pressure on the size and morphology of hydrothermal ZnO particles from ambient to supercritical conditions (T > 374 °C and P > 221 bar) using a unique continuous one-step process. Experiments were carried out from zinc nitrate (Zn(NO3)2) and potassium hydroxide (KOH) solutions in the ranges of 1–300 bar and 30–400 °C. The as-prepared particles of ZnO (flower, ellipsoid, and sphere) and e-Zn(OH)2 (polyhedral) sized from nano to micrometers were characterized by X-ray diffraction and electronic microscopy. The wulfingite phase (e-Zn(OH)2) was detected inside some powders especially at room temperature for higher pressu…